Method for reading a graphic pattern and acquiring its image

ABSTRACT

A method for reading a graphic pattern by illuminating the graphic pattern with at least two groups of light sources, each of the at least two groups of light sources having at least one light source operating according to an illumination cycle that comprises an illumination cycle-portion and a non-illumination cycle-portion. The light sources of one of the at least two groups of light sources are activated according to equal illumination cycles. The illumination cycles of the light sources belong to different ones of the at least two groups of light sources having a reciprocally different timing. Light is gathered from the light sources having been diffused by the graphic pattern on a sensor having a plurality of sensitive points. Light impinging on the plurality of sensitive points is converted, through a conversion cycle of the sensor, point by point into electric signals representative of single points of the graphic pattern, at a same time for all of the plurality of sensitive points.

RELATED APPLICATION

The present invention is a divisional of and claims priority from U.S.patent application Ser. No. 10/747,873, now U.S. Pat. No. 7,025,267, ofMichele Benedetti, filed Dec. 29, 2003, entitled “Method for Reading aGraphic Pattern and Acquiring Its Image.”

TECHNICAL FIELD

The present invention relates to the reading of a graphic pattern, thisterm indicating a one or two-dimensional graphic representation, suchas, for example, typically an optical code (barcode, two-dimensionalcode, colour code, etc.), or also any image that must be acquired.

BACKGROUND

The image acquisition of a graphic pattern is typically performedaccording to two main techniques: the laser scanning technique, whereinthe graphic pattern is scanned by a laser beam and the light diffusedpoint by point by the illuminated graphic pattern is gathered on asubstantially punctiform sensor and converted into electric signal, andthe CCD or CMOS techniques, wherein more points of the graphic patternare illuminated at the same time (on a total or partial area of thegraphic pattern, or on a line) and the light diffused by all theilluminated points is gathered on a one-or two-dimensional opticalsensor (of the CCD or CMOS type), capable of converting the lightimpinging it point by point into electric signals representing thesingle points of the graphic pattern, simultaneously for all sensitivepoints. The invention refers to the latter of the two techniques.

Theoretically, the graphic pattern can be illuminated just by theambient light, but specific illumination means are normally used,typically sets of approximately punctiform illuminating elements (suchas light diodes or LED), arranged in a one-dimensional array or in atwo-dimensional matrix, according to whether the reading is made bylines or by areas.

The light diffused by the illuminated portion of graphic pattern isgathered by an optical reception system (comprising lenses, diaphragms,mirrors and the like) and focused on the optical sensor. Finally, theoptical sensor comprises an array or an ordered matrix of singlepunctiform sensor elements, each gathering—at the same time as theothers—the light coming from the graphic pattern and converting it,always at the same time as the other punctiform elements, into a set ofelectric signals representing the optical characteristics of the singlepoints of the graphic pattern, thereby electrically reconstructing itsimage.

The problem of the unevenness of illumination on the area or line to beread is well known in the art. In fact, the central portion of the areaor of the line to be read is illuminated more intensely than theperipheral zones. This phenomenon, graphically shown in FIG. 2 in thecase of an array of four LEDs, is unavoidably associated to thegeometrical arrangement of the single illuminating elements and to thefact that each of them has an emission cone of a certain width. As itcan be easily seen in FIG. 2, the central zone receives illuminatingpower (light energy per area unit) from each of the various LEDs,whereas each of the two rightmost and leftmost zones only receivesilluminating power from the closest LED. The resulting distributioncurve of the illuminating power has a peak at about the centre anddecreases towards the ends. The same thing of course applies in case oftwo-dimensional illumination.

The result is that the peripheral zones of the graphic pattern are lessilluminated than the central zone, and so they diffuse less light,thereby producing an image of the graphic pattern that is distorted fromthe luminous intensity point of view.

Moreover, the problem of the illumination unevenness is made worse bythe uneven transmission of the optical reception system, which normallytends to transmit the illuminating power better in its central zone(close to the optical system axis) than in the peripheral zones. Atypical pattern of this phenomenon is shown in FIG. 3, which shows howthe power of the light composing the image decreases from the centretowards the edges.

The main effect of the phenomenon described is that the electric signalgenerated by the optical sensor will depend on the amount of lightreceived, and therefore it will have a variable amplitude pattern in thefield of view, according to the distance from the axis of the opticalreception system.

The overlapping of this unevenness can create serious problems for theproper acquisition of the image; for example, without correctivemeasures it may even occur that the noise gathered in the central zonehas the amplitude comparable to the signal collected in a peripheralzone. This amplitude unevenness can negatively affect the performance ofthe equipment for acquiring or reading the graphic pattern, in terms ofreduction of the aperture or of the depth of the reading field.

Such effects are further made worse as the reading or acquisitiondistance increases, since the electric signal becomes weaker.

Several approaches are known in the art to correct this situation.

According to a first approach, the problem is dealt with at the origin,by providing for the central LED to be piloted so as to produce a lessintense illumination compared to the peripheral ones. Examples of thisapproach can be found, for example, in U.S. Pat. No. 4,818,847 and5,144,117.

According to another approach that deals with the problem at the originas well, the spatial distribution of the LEDs and/or the orientation oftheir axes are not even; more precisely, the central LEDs are made to bemore spaced from one another or their axes are made to diverge towardsthe peripheral zones. An example of this approach is provided in U.S.Pat. No. 5,354,977.

Another known approach (EP-A-1205871), on the other hand, provides foran intervention during the signal electronic processing; that is, it isaccepted that the generated signal is affected by the above unevennessto intervene downstream by a gain system which is variable from zone tozone of the image.

SUMMARY OF THE INVENTION

The present invention aims at providing a different approach.

In an embodiment, the present invention provides a method for reading agraphic pattern. The graphic pattern is illuminated with at least twogroups of light sources, each of the at least two groups of lightsources having at least one light source operating according to anillumination cycle that comprises an illumination cycle-portion and anon-illumination cycle-portion. The light sources of one of the at leasttwo groups of light sources are activated according to equalillumination cycles. The illumination cycles of the light sources belongto different ones of the at least two groups of light sources having areciprocally different timing. Light from the light sources having beendiffused by the graphic pattern on a sensor having a plurality ofsensitive points is gathered. The light impinging on the plurality ofsensitive points, is converted, through a conversion cycle of thesensor, point by points, into electric signals representative of singlepoints of the graphic pattern, at a same time for all of the pluralityof sensitive points, the conversion cycle having at least an acquisitionstep. The acquisition step overlaps at least partially with theillumination step of the light sources of each of the groups of lightsources. The illumination cycle portion of the illumination cycle of anyof the groups of light sources has a temporal overlap with theacquisition step that is different from the temporal overlap of theillumination cycle portion of the illumination cycle of another of thegroups of light sources with the same acquisition step.

In an embodiment, the illumination cycles of all of the light sourcesare equal to one another, and wherein the illumination cycles of thelight sources of one of the at least two groups of light sources are nottimed with respect to the illumination cycles of the light sources of adifferent one of the at least two groups of light sources.

In an embodiment, the illumination cycle of each light source and theconversion cycle have a same period.

In an embodiment, each of the light sources is located a distance from aprivileged illumination zone, the method further comprising the step ofdividing the light sources into one of the at least two groups of lightsources according to the distance.

In an embodiment, for each of the light sources of one of the at leasttwo groups of light sources, the illumination cycle-portion overlaps thegathering step wherein a greater distance of the light sources of theone of the at least two groups of light sources from the privilegedillumination zone corresponds to a longer overlap.

In an embodiment, the conversion cycle comprises an acquisition step anda non-acquisition step regulated by a shutter, which, when activated,determines the non-acquisition step, and when not activated, determinesthe acquisition step.

In an embodiment, the conversion cycle comprises an acquisition step anda non-acquisition step and wherein the sensor operates according to asuccession of scanning steps having the same period, such scanning stepsbeing alternately used and not used, so that the scanning steps useddetermine the acquisition steps, whereas the scanning steps not useddetermine the non-acquisition steps.

In an embodiment, the at least two groups of light sources comprises twogroups.

In an embodiment, the illumination cycle-portion of the illuminationcycle of one of the two groups of light sources temporally correspondsto the non-illumination portion of the other one of the two groups oflight sources.

In an embodiment, the illumination cycle-portion of the illuminationcycle of one of the at least two groups of light sources temporallycorresponds to the non-illumination cycle-portion of another of the atleast two groups of light sources.

In an embodiment, the illumination cycle-portion of the illuminationcycle of one of the at least two groups of light sources temporallycorresponds to the non-illumination cycle-portion of all the other onesof the at least two groups of light sources.

In an embodiment, the reciprocally different timing is variable.

In an embodiment, the conversion cycle comprises an acquisition step anda non-acquisition step, the non-acquisition step overlaps at leastpartially with the illumination step of the light sources of at leastone of the groups of light sources.

In an embodiment the present invention provides a method for reading agraphic pattern. The graphic pattern is illuminated with at least twogroups of light sources, each of the at least two groups of lightsources having at least one light source operating according to anillumination cycle that comprises an illumination cycle-portion and anon-illumination cycle-portion. The light sources of one of the at leasttwo groups of light sources are activated according to equalillumination cycles. The illumination cycles of the light sources belongto different ones of the at least two groups of light sources having areciprocally different timing. The light from the light sources havingbeen diffused by the graphic pattern is gathered on a sensor having aplurality of sensitive points. The light impinging on the plurality ofsensitive points is converted, point by point, into electric signalsrepresentative of single points of the graphic pattern, at a same timefor all of the plurality of sensitive points, the conversion cyclecomprising at least an acquisition step. The acquisition step overlapsat least partially with the illumination step of the light sources ofeach of the groups of light sources.

Preferred solutions of the invention are indicated in the dependentclaims.

BRIEF DESCRIPTION OF THE DRAWING

Features and advantages of the present invention will appear moreclearly from the following detailed description of some of its preferredembodiments, made with reference to the attached drawings. In suchdrawings,

FIG. 1 is a diagram of an equipment for reading a graphic pattern;

FIG. 2 is a diagram showing the uneven illumination if the illuminatingsystem comprises an array of four LEDs;

FIG. 3 is a diagram showing the uneven transmission of the diffusedlight gathered by a typical optical reception system;

FIG. 4 is a diagram illustrating an embodiment of the invention, withilluminating system having two groups of light sources and opticalreception system provided with shutter;

FIG. 5 shows a particular case of the embodiment shown in FIG. 4;

FIG. 6 shows four examples of readings made in different conditions,according to the embodiment shown in FIG. 5;

FIG. 7 is a diagram similar to that of FIG. 4, showing anotherembodiment of the invention, with illuminating system having threegroups of light sources and optical reception system provided withshutter;

FIG. 8 is a diagram similar to that of FIG. 4, showing anotherembodiment of the invention, with illuminating system having two groupsof light sources and optical reception system without shutter;

FIG. 9 is a diagram similar to that of FIG. 4, showing anotherembodiment of the invention, with illuminating system having two groupsof light sources and optical reception system without shutter;

FIGS. 10 and 11 schematically show two illuminating systems to which itis possible to apply the embodiments of the invention of FIG. 4, FIG. 5,FIG. 8 and FIG. 9; and

FIG. 12 schematically shows an illuminating system to which it ispossible to apply the embodiment of the invention of FIG. 7.

DETAILED DESCRIPTION

The entire content of U.S. patent application Ser. No. 10/747,873,Michele Benedetti, filed Dec. 29, 2003, entitled “Method for Reading aGraphic Pattern and Acquiring Its Image,” and published as U.S. PatentApplication Publication No. US 2005/0082369 A1 on Apr. 21, 2005 ishereby incorporated by reference.

FIG. 1 shows a graphic pattern 1, for example a barcode, whose readingis performed by a reading equipment, only schematically shown andglobally indicated with reference numeral 2, which comprises anilluminating system 3, an optical reception system 4, a sensor 5, asignal processing unit 6, and a decoding unit 7. In the operation, thesingle points of the graphic pattern 1 illuminated by the illuminatingsystem 3 emit diffused light that is gathered by the optical system 4and carried onto sensor 5, where it is converted into electric signalsthat are first processed in the processing unit 6, and then decoded inthe decoding unit 7.

Sensor 5 consists of a plurality of flanked sensitive points, each ofwhich generates—at the same time as the other sensitive points—anelectric signal correlated to the characteristics of the light thatimpinges on it, and thereby to the characteristics of a correspondingpoint of the graphic pattern 1; the sensitive points can be arrangedalong a line (one-dimensional sensor), or in an area (two-dimensionalsensor). Typically, said sensor 5 will be of the CCD or CMOS type.

The optical system 4 can optionally be provided with a shutter 8, forexample of the mechanical type. According to an alternative preferredsolution, an electronic shutter 8′ can be directly associated to sensor5. The electronic shutter 8′ operates on the converted signal removing(resetting) the portion of signal converted starting from an initialinstant to a subsequent predetermined instant. The action of shutter 8or 8′ can be controlled by signals generated by a control unitintegrated in the same sensor, or separate from the sensor and containedin a suitable microcontroller (not shown).

The illuminating system 3 comprises a plurality of light sources 9divided into groups. More precisely, in the Figures from 10 to 12 theilluminating system and the light sources are indicated, besidesreference numerals 3 and 9, by a letter, according to the variant of theinvention considered; moreover, the light sources 9 are marked by afurther number to indicate the group they belong to.

So, FIG. 10 illustrates a one-dimensional illuminating system 3 a,comprising four light sources 9 a, divided into two groups according totheir distance from the optical axis X of the illuminating system 3 aitself; the two light sources 9 a, closer to the optical axis X belongto the first group, while the two light sources 9 a ₂ farther from theoptical axis X belong to the second group.

Similarly, FIG. 11 illustrates a two-dimensional illuminating system 3b, comprising eight light sources 9 b, divided into two groups accordingto their distance from the optical axis Y of the illuminating system 3 bitself; the four light sources 9 b ₁ closer to the optical axis Y belongto the first group, while the four light sources 9 b ₂, farther from theoptical axis Y, belong to the second group.

Finally, FIG. 12 illustrates a one-dimensional illuminating system 3 c,comprising six light sources 9 c, divided into three groups according totheir distance from the optical axis W of the illuminating system 3 citself; the two light sources 9 c ₁, closer to the optical axis W belongto the first group, the two light sources 9 c ₂ at an intermediatedistance from the optical axis W belong to the second group, while thetwo sources 9 c ₃, farther from the optical axis W, belong to the thirdgroup.

The illuminating system may also comprise an optical emission system(not shown) containing one or more lenses and possibly diaphragms, forfocusing the light emitted by the light sources 9.

The embodiment of the invention shown in FIG. 4 provides for thepresence of shutter 8 and for the light sources 9 to be divided into twogroups. With reference to said embodiment, each light source 9 a and 9 bis fed according to an illumination cycle 20, which comprises anillumination step 21 and a non-illumination step 22 following oneanother over time; the action of shutter 8 makes the conversion onsensor 5 occur according to a conversion cycle 23 (or scanning period),comprising a non-acquisition step 24 and an acquisition step 25 (orexposure time) following one another over time.

As shown in FIG. 4, the illumination cycle of the light sources 9 a ₁, 9b ₁, indicated with reference numeral 20 ₁, is equal to the illuminationcycle of the light sources 9 a ₂, 9 b ₂, indicated with referencenumeral 20 ₂, but the two cycles are not timed. This time differencecauses the light emitted by the light sources 9 a ₁, 9 b ₁ of the firstgroup to be partly unused or rejected, since it corresponds with thenon-acquisition step 24; on the other hand, the light emitted by thelight sources 9 a ₂, 9 b ₂ of the second group is fully used. Thenon-use of part of the light emitted by the light sources 9 a ₁, 9 b ₁of the first group therefore allows compensating both the illuminationunevenness and the transmission unevenness.

The amount of unused light emitted by sources 9 a ₁, 9 b ₁ of the firstgroup can be adjusted both by adjusting the time difference between thetwo illumination cycles 20 ₁ and 20 ₂, and adjusting the period of thenon-acquisition step 24, as well as adjusting the period of theillumination step 21.

A particular and interesting case is that shown in FIG. 5, wherein theillumination step 21 is equal to the non-illumination step 22, and thetwo cycles 20 ₁ and 20 ₂ are in phase opposition. The phase oppositionprovides for a single group of light sources always on, thereby reducingthe peak current absorbed by the illuminating system 3. FIG. 6 shows forthis case the effect of the variation of the period of the acquisitionstep 25 (or exposure time) on the signal produced by sensor 5; the fourcurves show how such period can be advantageously adjusted to reduce,cancel or even reverse the effects of the illumination and transmissionunevenness.

The embodiment of the invention shown in FIG. 7 provides for thepresence of shutter 8 and for the light sources 9 to be divided intothree groups. With reference to such embodiment, each light source 9 cis fed according to an illumination cycle 30, which comprises anillumination step 31 and a non-illumination step 32 following oneanother over time; the action of shutter 8 makes the conversion onsensor 5 occur according to a conversion cycle 33, comprising anon-acquisition step 34 and an acquisition step 35 following one anotherover time.

As shown in FIG. 7, the illumination cycle of the light sources 9 c ₁,indicated with reference numeral 30 ₁, is equal to the illuminationcycle of the light sources 9 c ₂, indicated with reference numeral 30 ₂,and to that of the light sources 9 c ₃, indicated with reference numeral30 ₃, but the three cycles are not timed. This time difference causesthe light emitted by the light sources 9 c ₁ of the first group and 9 c₂ of the second group to be partly unused or rejected, since itcorresponds with the non-acquisition step 34; on the other hand, thelight emitted by the light sources 9 c ₃ of the third group is fullyused. The non-use of part of the light emitted by the light sources 9 c₁ and 9 c ₁ of the first and of the second group therefore allowscompensating both the illumination unevenness and the transmissionunevenness.

As in the case of FIG. 4, also in this case the amount of unused lightemitted by the sources 9 c ₁ of the first group and 9 c ₂ of the secondgroup can be adjusted both by adjusting the time difference between theillumination cycles 30 ₁, 30 ₂ and 30 ₃ and adjusting the period of thenon-acquisition step 34, as well as adjusting the period of theillumination step 31.

The embodiment of the invention shown in FIG. 8 provides for no shutterand for the light sources 9 to be divided into two groups. Withreference to such embodiment, each light source 9 a and 9 b is fedaccording to an illumination cycle 40, which comprises an illuminationstep 41 and a non-illumination step 42 following one another over time;without shutter, potential acquisition (or scanning) steps follow oneanother on sensor 5, which are alternately rejected and used, so as tohave a conversion cycle 43, which comprises a non-acquisition step 44and an acquisition step 45, having the same period, that follow oneanother over time.

The situation is therefore similar to that discussed with reference toFIG. 4.

The embodiments illustrated in FIGS. 4-7 exhibit illumination cycleshaving the same period as the conversion cycle 23 (or 33) of the sensor.According to a variant, the conversion cycle 23 has a longer period thanthe illumination cycles. It is possible to select only a part of suchcycle corresponding to the period of the illumination cycles, therebydetermining the acquisition step 25 and the non-acquisition step 24.That is, in this case the method only works on the first illuminationcycle and the converted signal in the remaining portion of cycle 23 isrejected.

The embodiment of the invention shown in FIG. 9 provides for no shutterand for the light sources 9 to be divided into two groups. Withreference to such embodiment, each light source 9 a and 9 b is fedaccording to an illumination cycle 50, which comprises an illuminationstep 51 and a non-illumination step 52 following one another over time.The illumination cycles 50 differ in the two groups of sources 9 a and 9b and in particular, the period of the illumination step 51 ₂ forsources 9 a ₂ and 9 b ₂ (cycle 50 ₂) is longer than that 51 ₁ forsources 9 a ₁, 9 b ₁ (cycle 50 ₁). Conversion cycles 53 follow oneanother on sensor 5 with a period corresponding to that of theillumination cycles 50; the conversion cycles 53 only comprise anacquisition step 55 and no non-acquisition step. The effect of thissolution is that the quantity of light emitted by the sources of thesecond group 9 a ₂ and 9 b ₂ (given by the integral of the respectivewaveform represented) is greater than that emitted by the first group,therefore compensating both the illumination unevenness and thetransmission unevenness.

According to a variant, the conversion cycle 53 has a longer period thanthe illumination cycles 50 ₁ and 50 ₂. It is possible to select only aportion of such cycle corresponding to the period of the illuminationcycles, thereby determining the acquisition step 55.

The embodiments shown in FIGS. 4-8 exhibit equal illumination cyclesbetween the different groups of sources.

However, it is also possible to differentiate such cycles from oneanother, for example by increasing the period of the illumination stepof a given group of sources with respect to another one, similarly towhat described for the embodiment of FIG. 9. In this way it is possibleto increase the correction effect already produced by the non-use ofpart of the light emitted by a predetermined group of sources.

Moreover, it is possible to increase the effect given by the methodsillustrated above by also differentiating the intensity of the sourcesfeeding current for the different groups of sources, for examplesuitably increasing it in cycles 20 ₂ of FIGS. 4 and 5, 30 ₃ of FIG. 7and 40 ₂ of FIG. 8.

Finally, in all of the above embodiments, it is possible to vary theintensity of the sources feeding current according to the distance ofthe graphic pattern.

The invention has been described in various embodiments with referenceto the typical case in which the illumination and transmissionunevenness occurs as shown in FIGS. 2 and 3, with peak (or privilegedillumination zone) in the proximity of the optical axis, and minimum atthe periphery. However, it can be advantageously applied in all cases ofunevenness occurring for any reason, wherever the privilegedillumination zone; it will be sufficient to select the groups of lightsources in the most appropriate manner according to the specificunevenness to compensate.

Various modifications and alterations of this invention will be apparentto those skilled in the art without departing from the scope and spiritof this invention. It should be understood that this invention is notlimited to the illustrative embodiments set forth above.

1. A method for reading a graphic pattern, comprising the steps of:illuminating said graphic pattern with at least two groups of lightsources, each of said at least two groups of light sources having atleast one light source operating according to an illumination cycle thatcomprises an illumination cycle-portion and a non-illuminationcycle-portion; wherein said light sources of one of said at least twogroups of light sources are activated according to equal illuminationcycles; wherein said illumination cycles of said light sources belongingto different ones of said at least two groups of light sources have areciprocally different timing; gathering light from said light sourceshaving been diffused by said graphic pattern on a sensor having aplurality of sensitive points; and converting, through a conversioncycle of said sensor, said light impinging on said plurality ofsensitive points, point by point, into electric signals representativeof single points of said graphic pattern, at a same time for all of saidplurality of sensitive points, said conversion cycle comprising at leastan acquisition step; wherein the acquisition step overlaps at leastpartially with the illumination cycle portion of the light sources ofeach of the groups of light sources; wherein said illumination cycleportion of said illumination cycle of any of said groups of lightsources has a temporal overlap with the acquisition step that isdifferent from the temporal overlap of the illumination cycle portion ofsaid illumination cycle of another of said groups of light sources withthe same acquisition step.
 2. The method according to claim 1 whereinsaid illumination cycles of all of said light sources are equal to oneanother, and wherein said illumination cycles of said light sources ofone of said at least two groups of light sources are not timed withrespect to said illumination cycles of said light sources of a differentone of said at least two groups of light sources.
 3. The methodaccording to claim 1 wherein said illumination cycle of each lightsource and said conversion cycle have a same period.
 4. The methodaccording to claim 3 wherein each of said light sources is located adistance from a privileged illumination zone, said method furthercomprising the step of dividing said light sources into one of said atleast two groups of light sources according to said distance.
 5. Themethod according to claim 4 wherein, for each of said light sources ofone of said at least two groups of light sources, said illuminationcycle-portion overlaps said gathering step wherein a greater distance ofsaid light sources of said one of said at least two groups of lightsources from said privileged illumination zone corresponds to a longeroverlap.
 6. The method according to claim 3 wherein said conversioncycle comprises an acquisition step and a non-acquisition step regulatedby a shutter, which, when activated, determines said non-acquisitionstep, and when not activated, determines said acquisition step.
 7. Themethod according to claim 3 wherein said conversion cycle comprises anacquisition step and a non-acquisition step and wherein said sensoroperates according to a succession of scanning steps having said sameperiod, such scanning steps being alternately used and not used, so thatsaid scanning steps used determine said acquisition steps, whereas saidscanning steps not used determine said non-acquisition steps.
 8. Themethod according to claim 1 wherein said at least two groups of lightsources comprises two groups.
 9. The method according to claim 8 whereinsaid illumination cycle-portion of said illumination cycle of one ofsaid two groups of light sources temporally corresponds to saidnon-illumination portion of the other one of said two groups of lightsources.
 10. The method according to claim 1 wherein said illuminationcycle-portion of said illumination cycle of one of said at least twogroups of light sources temporally corresponds to said non-illuminationcycle-portion of another of said at least two groups of light sources.11. The method according to claim 1 wherein said illuminationcycle-portion of said illumination cycle of one of said at least twogroups of light sources temporally corresponds to said non-illuminationcycle-portion of all the other ones of said at least two groups of lightsources.
 12. The method according to claim 1 wherein said reciprocallydifferent timing is variable.
 13. The method according to claim 1wherein said conversion cycle comprises an acquisition step and anon-acquisition step, the non-acquisition step overlaps at leastpartially with the illumination cycle portion of the light sources of atleast one of the groups of light sources.
 14. A method for reading agraphic pattern, comprising the steps of: illuminating said graphicpattern with at least two groups of light sources, each of said at leasttwo groups of light sources having at least one light source operatingaccording to an illumination cycle that comprises an illuminationcycle-portion and a non-illumination cycle-portion; wherein said lightsources of one of said at least two groups of light sources areactivated according to equal illumination cycles; wherein saidillumination cycles of said light sources belonging to different ones ofsaid at least two groups of light have a reciprocally different timing;gathering light from said light sources having been diffused by saidgraphic pattern on a sensor having a plurality of sensitive points; andconverting, through a conversion cycle of said sensor, said lightimpinging on said plurality of sensitive points, point by point, intoelectric signals representative of single points of said graphicpattern, at a same time for all of said plurality of sensitive points,said conversion cycle comprising at least an acquisition step; whereinthe acquisition step overlaps at least partially with the illuminationcycle-portion of the light sources of each of the groups of lightsources.